1. Field of the Invention
The present invention relates to an apparatus for optical transmission and a method therefor, for transmitting an amplified light signal via a transmission path. In particular the invention relates to an apparatus for optical transmission and a method therefor, where release to the surroundings of a high output light signal is prevented in the case where a connector for connecting an optical fiber becomes disconnected or the optical fiber becomes damaged.
2. Description of the Related Art
For example, with image distribution systems such as optical CATV systems, in order to provide a service to several thousand subscribers with a single system, it is required to branch a high output light signal of a high optical power level into a plurality to transmit to the respective subscribers. To obtain such a high output light signal, in general an optical amplifier or the like is used.
With a system for handling such a high output light signal, then for example, in a case such as where a connector connecting between an output port outputting a light signal and a transmission path becomes disconnected, there is the possibility of the high output light signal being released to the surroundings. To prevent this release of the high output light signal, conventionally countermeasures are adopted such as having a construction where for example the Fresnel reflection light generated at the end face of the opened output port due to disconnection of the connector is detected, and the amplifying operation of the optical amplifier is controlled to stop or attenuate the light output so that the high output light signal is not released to the surroundings.
As a conventional apparatus incorporating such a function to control the operation of an optical amplifier at the time of detecting the occurrence of an abnormality, to prevent the release to the surroundings of the high output light signal, there is for example the disclosures of Japanese Unexamined Patent Publications Nos. 60-101503, 5-291667, 5-291669, and 6/244805.
However, with systems such as with an image distribution system wherein light signals from a single optical amplifier are multiply branched and used, then for example, at the time of enlargement of the circuit, the terminator attached to the unused port is removed, and the additional circuit connected. In such a case, with the conventional technique, the reflected light at the end face of the output port which has been opened by removing the terminator is detected, and the amplifying operation of the optical amplifier is controlled to stop or attenuate the light output. Consequently, there is the problem that all of the circuits which receive the light signals from the amplifier are shut-off (normal light signal output is shut-off).
In order to solve the abovementioned problem at the time of circuit enlargement, then for example, a measure has been previously adopted which adjusts the detection level of the reflection light so that if only one or two ports are opened, the amplifying operation of the optical amplifier is not limited. If this measure is adopted however, there is the problem that in the case where only one or two ports are opened, high output light signals are released from the opened port.
Furthermore, in the case of the conventional technique where the reflected light is detected, to thereby judge the occurrence of an abnormality in the transmission path, then for example, when an optical fiber is damaged close to a subscriber, the reflected light is attenuated due to the loss between the output port and the damage point, and hence detection of the reflected light becomes difficult. On the other hand, when an optical fiber is damaged close to an output port, detection of the reflected light becomes difficult depending on how this is cut (for example when the breakage is at an incline etc.). Due to this situation where the necessary detection function for the reflected light does not operate normally, a high output light signal is released from the damaged point.